<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background-color: #000;
        }
        
        .x {
            background-color: rgb(0, 0, 255);
        }
        
        .trans {
            background-color: rgb(255, 0, 0.5);
        }
    </style>
</head>

<body>
    <div class="x"></div>
    <div class="trans"></div>
</body>

</html>